The most frequent malignant tumors of CNS are metastases, representing a significant cause of morbidity and mortality in patients with cancer from various neoplasms, mainly from lung cancer, breast cancer, and melanoma [
55]. The TME where BM had spread includes cancer and noncancerous cells (i.e., endothelial cells, pericytes, fibroblasts, and immune cells) [
56]. The process in which tumor invasion is directed toward the nervous tissue is mediated by different cellular interactions and the brain ME [
57]. Reportedly cancer cells activate astrocytes at the metastatic site triggering positive feedback to adapt to the new ME and initiate colonization [
17].
The process of cell arrival or nesting in the brain was previously considered a random process. However, the expression of the alpha-2,6, sialyltransferase gene in CTC is associated with a predilection toward BBB, as its protein serves as an adhesion molecule [
57]. In breast cancer, the
N-acetylgalactosaminidase α-2,6-sialyltransferase 5 (ST6GALNAC5) has been identified as a facilitator of tumor cell/brain endothelial adhesion [
46]. In melanoma, the membrane-bound melanotransferrin correlates with this ability [
58], and in small cell lung cancer, the Rho kinase signaling, involved in the intracellular junction disruption, has been activated in this transendothelial migration [
59]. In breast cancer, the chemokine receptor CXCR4 and its ligand CXCL12, also known as the stromal cell-derived factor 1-α, increase vascular permeability and activates the PI-3K/AKT pathway [
60]. Factors produced by CTC that predominately aim toward BBB include α-crystallin, ADAM 8 (disintegrin and metalloproteinase domain-containing protein 8), MMP-1, PLEKHA5 (pleckstrin homology domain-containing family A member 5), PT1C (carnitine palmitoyl-CoA transferase), neuroserpin, serpin B2, and CTSS (cathepsin S) [
61].
Transmigration
Transendothelial migration (transmigration) is the process of cancer cells traversing BBB. Factors associated with transmigration include cyclooxygenase COX2 (PTSG2), CXCL12/CXCR4, ST6GALNAC5, CTSS, MMP-1, α-crystallin, angiopoietin-like 4 (ANGPTL4), EGFR ligand heparin-binding EGF (HB-EGF), and VEGF [
62]. COX2 activity produces prostaglandin, which increases the BB permeability [
63]. Cancer cells produce miRNA-181c, which dysregulates the dynamics of the intracellular actin of BBB via the downregulation of its target gene PDPK1, thus favoring transmigration in vitro and in vivo [
61]. Other targets of BBB that serve as tropism factors for CTCs include the expression of E-selectin at brain endothelial cells, which might indicate the site where successful BM occurs [
64], and the cell adhesion molecule L1 (L1CAM) [
65]. A premetastatic metabolic niche is either required or prepared by CTCs. CTCs secrete miRNA 122 and suppress glucose uptake by neurons and astrocytes. The inhibition of miRNA-122 decreases the incidence of metastases [
66].
After traversing BBB, cancer cells secrete various cytokines, chemokines, and mediators, particularly IL-1β, TNF-α, IFN-γ, CCL2, CXCL8, and COX2 [
4]. Cancer cells can secrete MMP and other proteases to break down the basement membrane, similar to that occurring in metastases to other sites, and can induce endothelial cell apoptosis [
67].
IFN-mediated signaling pathway is essential for antitumor immune response. IFN is produced by numerous immune cells to directly modulate the ME. They also alter gene expression in cancer cells, IFN-stimulated genes (ISGs), and include a family of TFs (IRF3, IRF5, and IRF7), which are the dominant regulators of ISG expression. Metastatic cancer cells downregulate the IFN type I response as a mechanism to promote immune prevention at the metastatic site; therefore, a lack of type I IFN response increases the risk of metastases [
33].
In addition to expressing L1CAM and traversing paracellularly into BBB, cancer cells secrete serpins that inhibit the activation of plasmin-mediated astrocytes, thereby preventing the secretion of the pro-apoptotic FAS ligand [
68,
69].
Astrocytes maintain homeostasis in the brain ME. Branching astrocytic processes cover most cellular components of CNS, including BBB. Reactive astrocytes release interleukins and upregulate several survival genes in breast cancer cells, such as GSTA5, BCL2L1, and Twist1, promoting resistance to chemotherapy [
70]. Metastatic cells then take advantage of the protective function of astrocytes by communication through gap junctions, causing astrocytes to generate survival factors such as IL-1β or CCL2, and by maintaining BBB, which generates another chemoresistance mechanism of BM [
71,
72].
The arrest and extravasation of cancer cells result in a strong local activation of astrocytes, detected by the upregulation of glial fibrillary acidic protein (GFAP) as well as by hyperdilation of astrocyte processes [
73]. In addition to the elevated expression of GFAP, some astrocytes associated with cancer cells simultaneously regulate the expression of nestin, another marker of reactive astrocytes [
74].
A significant consequence of the activation of astrocytes is their ability to secrete factors such as MMP-9. MMP-9 can directly affect the invasion of cancer cells and has proangiogenic and growth-promoting functions in brain tumors through the release of ECM growth factors [
73]. A strong expression of MMP-9 and a strong upregulation of the MMP-9 protein in the vicinity of the extravasation of cancer cells, associated with activated astrocytes surrounding the tumor cells, have been observed [
73]. Moreover, cancer cells interact with neural stem cells by overexpressing BMP-2, which signals the cells to differentiate into astrocytes [
75].
Cancer cells use neurotransmitters as oncometabolites. For example, GABA is used by cancer cells to form NADH [
76]. Neurotrophins are endogenous neuron growth factors [such as NT-3, NT-4, nerve growth factor, brain-derived neurotrophic factor (BDNF)]. Cancer cells express the neurotrophin receptor TrkB, which is selectively activated by BDNF, with a possible interaction of HER2+ with TrkB currently being investigated [
75]. Microglia form a part of the mononuclear phagocytic system and can respond to the invasive cancer cells via cytotoxic mechanisms. The WNT pathway keeps the microglia active and activates the proliferation of cancer cells. Furthermore, cancer cells express NT-3, which increases the metastatic potential, potentially via microglia-mediated mechanisms [
51].
Extravasated cancer cells have to stay in close physical contact to the abluminal surface of the blood vessels to remain viable [
77]. VEGF-A influences multiple steps of the metastatic cascade, and anti-VEGF-A therapy can induce long-term dormancy of small, perivascular lung carcinoma metastases [
78]. Metastasis is a process rather than a simple endpoint, wherein the arrest of the cancer cells occurs at vascular branch points, cancer cells have to remain close to microvessels, and perivascular growth by early angiogenesis is predominant in lung cancer and by vessel co-option in melanoma [
77]. In comparison, breast cancer cells that reach the brain exhibit a GABAergic phenotype similar to neuronal cells, They can use GABA as an energy metabolite and increase cell survival [
79,
80].