Table 4.

Cell surface molecules associated with tumor cell extravasation across the blood–brain barrier and the formation of brain metastases

Cell surface molecules	Findings	Primary tumor	Refs
Integrins	Both in vivo and in human brain lesion specimens, tumor micrometastases localize to the neurovascular BL and co-opt existing vessels. Blockade of β1 integrin-mediated adhesion in tumor cells prevents adhesion to the neurovascular BL and decreases metastatic colonization and outgrowth in vivo	Breast cancer	[191]
	In vitro and in vivo activation of αvβ3 causes the continuous post-transcriptional upregulation of VEGF, promoting the growth of metastatic brain lesions, but not the growth of the primary tumor		[192]
	Invasiveness of brain metastatic tumor cells influenced by the combined effects of αv integrin and HER2, with αv knockdown disrupting HER2 localization and reduced tumor cell motility in vitro and decreased brain invasiveness in vivo		[193]
	The anti-αv monoclonal antibody, intetumumab, decreases brain metastases and increase survival in an in vivo animal model		[194]
	Increased β4 signaling disrupts brain EC junctional complexes by inducing HER2-dependent expression of VEGF		[195]
	Increased expression level of α4ß1 in brain metastatic tumor cells, both in vitro and in vivo. Receptor for α4ß1, VCAM1, widely expressed on the EC surface and as early as 5 days after intracardiac induction in vivo. Blockage of the α4 subunit significantly reduced in vivo metastatic seeding		[175]
	Rab11b-mediated cellular recycling of integrin β1 regulates brain metastatic breast cancer outgrowth, modulating interaction with ECM, facilitating mechanotransduction-activated survival signaling		[196]
	αB-crystallin expression in primary tumor associated with poor survival after brain metastasis. Overexpression of αB-crystallin enhances—and silencing inhibits—adhesion of tumor cells to ECs in vitro. Mechanism of adhesion partially achieved through α3β1 integrin. Brain metastases in vivo were increased or reduced by overexpressing or silencing αB-crystallin, respectively		[197]
	The antipsychotic agent, penfluridol, reduces the expression of integrin α6 and integrin β4 on tumor cells in vitro. Penfluridol treatment significantly inhibited the growth of brain metastases in vivo. Penfluridol-treated tumors demonstrated decreased integrin β4 and increased apoptosis		[198]
	High expression of αvβ5 on vascular structures and tumor tissue in brain lesions associated with high hypoxia inducible factor 1α (HIF)-1α indices (related to tumor survival in hypoxic conditions). Brain lesions with a αvβ3 expression pattern correlated with low Ki-67 proliferation indices and favorable survival times	Lung cancer	[201]
	Tumor cells with greater expression of α3 demonstrated greater ECM attachment, migration, and proliferation in vitro. Blocking α3β1 in vivo significantly decreases brain metastasis		[202]
	Over 90% of tested patient brain metastases expressed α4β1. In vitro antibody ablation of α4β1 reduces tumor cells arrest and BBB disruption	Melanoma	[203]
	Expression of αvβ6 significantly higher in brain metastases with well-demarcated growth compared to vascular co-option and diffuse infiltration. Expression of αv in patient brain metastatic lesions significantly higher than in the primary tumor	Multiple types	[199, 200]
ALCAM	High tumor ALCAM expression and increased ALCAM endothelial expression in vivo during early metastasis seeding. Anti-ALCAM antibodies significantly decreased brain metastasis seeding in vivo	Breast cancer	[175]
	ALCAM expression significantly increased in patient brain metastases, with increased expression in primary tumor and brain metastases associated with shortened survival. In vitro ALCAM knockdown reduces tumor cell adhesion to cerebral endothelial cells. ALCAM knockdown tumor cells produced reduced brain metastatic tumor seeding in vivo	Lung cancer	[176]
	Proof-of-concept study for ALCAM-targeting MRI contrast agent using in vivo brain metastasis model. ALCAM-targeting contrast agent was able to detect brain micrometastases from lung, breast and skin cancer	Multiple types	[177]
VCAM1	VCAM1 is expressed in human brain metastases and micrometastases. Targeted MRI contrast agent for VCAM1 revealed upregulated expression in brain metastases 5 days after induction in vivo	Breast cancer	[210]
	Induction of cerebrovascular inflammation significantly increases brain expression of VCAM1 in vivo. Intracardiac injection of tumor cells in mice with induced cerebrovascular inflammation increases brain metastatic burden, however blocking VCAM1 before tumor cell injection prevents this increase		[208]
	Anti-VCAM1 antibody produces significant reduction in brain metastatic burden and increased overall survival in vivo		[211]
	Increased VCAM1 expression and microvessel density at the boundary of tumor tissue and surrounding brain tissue in animal xenograft model. Similar results were observed in human brain metastasis specimens		[209]
	Anti-VCAM1 antibodies partly inhibit tumor cells adhesion to brain ECs	Prostate cancer	[185]
	VCAM-1 expression in human brain metastasis specimens, across lung, breast and skin cancer. Targeted MRI contrast agent for VCAM1 revealed upregulated VCAM1 in tumor-associated microvessels	Multiple types	[207]
PECAM1	PECAM1 associated with a highly brain metastatic tumor cell model	Breast cancer	[178]
L1CAM	L1CAM mediates vascular co-option by brain metastases, which is promoted by serpins. L1CAM also activates YAP via integrin β1 and integrin-linked kinase, facilitating metastatic outgrowth	Multiple types	[179, 180]
Melano-transferrin	Tumor cell ability to extravasate across the BBB correlates with tumor cell-surface expression of melanotransferrin in vitro. Application of anti-melanotransferrin antibody significantly reduced the development of brain metastases in vivo	Melanoma	[181]
E-Selectin	E-selectin promotes adhesion and extravasation of estrogen receptor(–)/CD44(+) tumor cells, but not estrogen receptor(+)/CD44(−/low) tumor cells in vitro. In estrogen receptor(–) breast cancer, CD44(+) tumor cells are found in high quantities in human brain lesion specimens. In vivo application of an E-selectin antagonist significantly reduced brain metastases	Breast cancer	[182]
	Concomitant high expression of BST-2 with CD15s (E-Selectin binding partner) in ER-negative tumors from patients is associated with higher risks of liver and brain metastasis and decreased survival rate		[183]
	Tumor cells and tumor-secreted factors increase E-selectin expression on cerebral endothelial cells in vitro. Endothelial glycocalyx degradation correlates with increased tumor cell adhesion. Inhibition of E-selectin attenuates tumor cell adhesion	Lung cancer	[155]
	TNF-α associated with increased E-selectin on cerebral ECs. Immunoblocking of the E-selectin binding partner CD15 on tumor cells reduces adhesion to cerebral ECs. Both CD15 and E-selectin are expressed in patient brain metastatic lesions. Overexpression of CD15/CD15s increases tumor cell adhesion to the E-selectin on cerebral ECs, increasing the disruption of cerebral endothelial cell monolayers. Knockdown of FUT4/ FUT7, which code for CD15/CD15s, prevents in vitro BBB disruption. Overexpression of FUT4/ FUT7 in non-metastatic tumor cells increases metastatic phenotype		[156, 184]
	Tumor cell adhesion significantly increased by upregulated TNF-α, with increased E-selectin expression on cerebral ECs. Anti-E-selectin antibodies partly inhibit adhesion of tumor cells to brain ECs. Human tumor cells derived from brain metastases express the E-selectin ligand, E-selectin ligand-1	Prostate cancer	[185, 186]
E-cadherin/ N-cadherin	Increased E-cadherin expression in metastases to the liver, lung and brain compared to the primary tumor. Ectopic expression of E-cadherin causes tumor cells with mesenchymal phenotype to revert to epithelial phenotype in vitro. Tumor cells with a mesenchymal phenotype injected into primary tumor site express E-cadherin after metastasizing in vivo. E-cadherin linked to resistance to ionizing radiation and chemotherapy	Breast cancer	[187, 190]
	N-cadherin expression highly predictive of brain metastasis–free survival. Low E-cadherin expression in patients associated with increased risk of developing brain metastasis	Lung cancer	[188, 189]