*2.1.4 Extravasation*

Cancer cells extravasate from a vascular lumen into tissues such as the lung, liver, and brain by passing through the endothelial cell and pericyte layers. Extravasation is comparable to intravasation in that it is morphologically similar to invadopodia but mechanistically different. Certain cell types in the primary tumor microenvironment, such as TAMs [34], can initiate intravasation, but these same cells do not have the same promoting function in the extravasation process of disseminated CTCs. Indeed, macrophage phenotype and function differ between primary and metastatic tumor locations. For example, macrophage seeding at distant places is VEGFR+, CCR2+, CXCR4-, Tie2-, and the subpopulation of macrophage at perivascular macrophage is phagocytic [67].

CTCs must overcome the physical barriers of the microvascular wall to extravasate. According to some studies, primary tumors have been shown to release substances that interfere with these distant microenvironments and cause vascular hyperpermeability. Secreted protein angiopoietin-like-4 (Angptl4), as well as the pleiotropically active proteins like NOX4, MMP-1, and MMP-9, disrupt pulmonary vascular endothelial cell-cell junctions, allowing colorectal cancer cells to extravasate into the lungs [68]. Angiopoietin2 (Angpt2), MMP-3, MMP-10, placental growth factor, and VEGF, all of which are secreted by many types of primary tumors, can induce pulmonary hyperpermeability prior to the arrival of cancer cells in the lungs, allowing CTCs to extravasate more easily [69, 70]. Finally, by secreting VEGF, inflammatory monocytes recruited to pulmonary metastases via CCL2-dependent processes increase breast cancer cell extravasation in the lung [71].

Interestingly, whereas Anglptl4 improved the extravasation of breast carcinoma cells in the lung, it did not increase the extravasation or intravasation efficiency of these same breast cancer cells in the bone [72]. As a result, Anglptl4 selectively and only increases extravasation within the lung tissue environment.

#### *2.1.5 Colonization and metastatic growth*

Cell-nonautonomous mechanisms required to transform a foreign microenvironment into a more friendly niche may be required for disseminated tumor cells to emerge from hibernation and begin active proliferation. For example, the growth of other inactivated disseminated tumor cells might need to stimulate BMDCs to enter the circulation system, as well as the followed recruitment of these cells to the metastatic location; in some situations, the process may be activated through systemic signals such as osteopontin (OPN) or SDF-1 produced by cancer cells, [73, 74].

Alternatively, because the body is in a constant state of homeostasis, dormant cancer cells could continue to proliferate without a net increase. The reasons driving such high rates of attrition are unknown, however, a lack of disseminated tumor cells to initiate neoangiogenesis has been hypothesized as one possible explanation. Prostate tumor cell-secreted prosaposin (Psap) may limit metastatic colonization by increasing the expression of the anti-angiogenic factor thrombo-spondin-1 in stromal cells, which is consistent with this theory [75]. Angpt2, on the other hand, promotes the metastatic colonization of breast and pancreatic cancer by improving the infiltrating capability of myeloid cells to support the vascularization of metastatic nodules [76].

Numerous genes promote the metastatic colonization of cells in breast cancer to bone, lung, brain, or liver, which have recently been discovered. These genes are able to adapt and overcome incompatibilities between the special development procedure of disseminated cancer cells and the demands from foreign tissue milieu, in parallel, researchers come up with the idea that these genes could control organ-specific metastatic tropism. The osteoclastic cytokine IL-11 is an excellent example of this, IL-11 works through a receptor activator for nuclear factor kB (RANK), which disrupts the normal crosstalk between osteoclasts and osteoblasts [77]. Moreover, it strengthens metastatic tumor growth in breast cancer and osteolysis by JAK1/STAT3/c-myc signaling pathway rather than in a RANKL-dependent manner [78].

Similarly, the Notch ligand Jagged1 enhances the osteolytic bone metastases in breast cancer cells by boosting osteoclast activity through IL-6 released by osteoblasts [79]. By encouraging osteoclast action, IL-11 and Jagged1 are able to cause osteolysis and release the rich deposits of growth factors from the bone matrix. The fact that genes identified as candidate mediators of breast cancer cell metastatic colonization in bone, lung, brain, or liver show very little overlap, illustrates the idea that different tissue microenvironments are needed to be organ-specific for metastatic colonization.

#### **2.2 Routes of cancer metastasis**

#### *2.2.1 The circulatory system*

Despite the fact that lymphatic diffusion of cancer cells is a key prognostic marker for cancer progression, spreading through the blood circulation seems to be the main mechanism of dispersal of metastatic carcinoma cells. Based on intravital imaging studies, tumor cells can travel toward blood arteries. Li et al. injected metastasized breast cancer cells in mice and found that these cells move toward arteries, illustrating that metastatic cells have the ability of directional migration toward blood streams [80]. Morphologically, compared with non-metastatic cells, metastatic cells are more round,
