At some point during its journey circulating through blood vessels, CTCs face another challenge: getting out of circulation. As previously discussed, many CTCs become lodged in capillaries if not extravasated. Extravasation refers to the essential process CTCs take to exit circulation and enter target tissues to spread at a distant site. Successful extravasation is a two step process, requiring that the CTC properly 1) adhere and 2) translocate with the endothelial wall.
The first step of extravasation requires that a CTC properly adhere with the endothelial wall. Adhesion relies on mutual affinities between proteins on the surface of CTCs and the endothelial wall. Some proteins we will discuss on CTCs include primary extracellular receptors (such as integrins and chemokine receptors) and secondary proteins on platelets of microthrombi.
CTCs extravasate out of the circulation by homing to the endothelial surface receptors using integrins. Integrins are proteins mediating cell-cell or cell-ECM adhesions (1-6). Integrins bind to fibrinogen, which is a common glycoprotein found in the extracellular matrix (3-6). Integrin-fibrinogen binding is a key player in extravasation-related adhesion, and has become a related target for cancer therapies. In addition, some integrins on CTCs have been shown to target basement membrane receptors, allowing CTCs to pass through gap junctions in the endothelium, providing an easy path for extravasation (5,6). This phenomenon is observed in lung metastasis, where α3β1 integrins expressed on CTC bind to laminin-5 located onthe vascular basement membrane (1).
Chemokine receptors also contribute to specific binding or preference of CTCs to metastasize to certain tissues rather than others (3,6). Under these circumstances, CTCs are more likely to metastasize to areas of the body that present the ligands for the receptor. An example of this is found in metastasizing breast cancer cells, which have been discovered to possess chemokine receptors CXCR4 and CCR7. CXCR4 expression target to CXCL12/ SDF-1alpha ligands on tissues, such as in lungs (1,6). Therefore, specific receptor-ligand binding pairs emphasize the phenomenon of heterogeneity in CTC homing during metastatic dissemination from primary tumors. Furthermore, understanding these interactions will help provide us with greater knowledge on how to prepare targeted therapies for each metastatic cancer type. It has also been implicated that CTCs may make use of this specificity for 'preparing' their microenvironment in the targeted tissue (see pre-metastatic niche).
CTCs that have already formed microthrombi will be encapsulated in platelets, thus restricting the presentation of receptors and slightly altering the CTC properties. They are larger than regular CTCs due to the extra layers of platelets, and may be able to be trapped in arterioles and not just capillaries. These properties may actually provide more potential sites for extravasation. Platelets also strongly bind to the surface of injured blood vessels which have exposed basement membranes (2-3). In this way platelets could act as homing signals for CTCs by finding gaps in the epithelium in the same way a specific integrin might.
The next step for extravasation to occur is to translocate the cancer cell pass the endothelium. The process of moving though the endothelium of the vessel is called transendothelial migration (TEM). TEM can happen in different ways. One method TEM occurs is by brute force. Rapidly dividing mass of cells could mechanically burst the vessel by applying more pressure than the walls can handle (2,4,7).The more refined approach involves the CTC breaking endothelial cell junctions. Cell junctions hold neighbouring cells together directly, or can indirectly hold cells together by attachment to the extracellular matrix. It is thought that cancer cells can break through this tightly linked fence of cells by initiating angiogenic signaling pathways like VEGF (4,5). Endothelial cells respond to this factor by activating Src kinases, which results in the disruption of cell junctions. Once these cell junctions are weakened, the cancer cell can then induce endothelial retraction through structural rearrangements of the cytoskeleton. This action gives the cancer cell more space to pass through the endothelium (3,4). The cancer cell can then actively push between the endothelial cells in a process called intercalation (3).
Some of the most important factors in TEM are the Rho family GTPases, which help to regulate the cytoskeleton and the turnover of cell-cell and cell-ECM adhesions (3). The consequences of their actions result in the control of actin turnover and polymerization for intercalation. In addition, these Rho GTPases have been discovered to be regulated by Cdc42, and is so central that Cdc42 depletion sufficiently decreases metastasis formation by disrupting the steps involved in TEM (3).
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