Table 1.

Proteases associated with tumor cell extravasation across the blood–brain barrier and the formation of brain metastases

Proteases	Findings	Primary tumor	Refs
ADAM8	Regulates MMP9 expression in tumor cells, with inhibition leading to reduced trans-endothelial migration in an in vitro BBB model	Breast cancer	[128]
ADAM9	ADAM9 expression is greater in highly brain metastatic tumor cells compared to bone-metastatic or primary tumor cells, with more invasive potential, increased adhesion capacities, and greater expression of α3β1 integrin	Lung cancer	[127]
Cathepsin S	Associated with decreased brain metastasis-free survival. Produced by macrophages and tumor cells. Facilitates BBB extravasation through proteolytic degradation JAM-B. Depletion via inhibitors significantly reduces brain metastasis in vivo	Breast cancer	[126]
Heparanase	Correlates with the brain metastatic potential of tumor cells in patients	Breast cancer	[98]
	EGFR-triggered nucleolar localization of heparanase produces DNA topoisomerase-I modulation and enhances brain metastatic proliferation		[97]
	Active and inactive heparanase enhances EGFR phosphorylation via Src, increasing tumor migration and proliferation, correlating with head and neck tumor progression		[96]
	Downregulation of miR-1258, involved in the expression of heparanase, inversely correlates with metastatic invasion to the brain		[99]
	Accumulates around blood vessels in brain metastatic melanoma specimens	Melanoma	[100]
	Increases the invasion potential of tumor cells across an in vitro BBB model		[101]
	Overexpression of the neurotrophin receptor p75NTR on tumor cell surface links to the invasive properties of heparanase		[102, 103, 105, 106]
	Supra-additive levels detected when brain metastatic tumor cells were incubated with endothelial cells and astrocytes		[104, 107]
MMPs	Increased MMP1 expression in early circulating tumor cells with EMT phenotype	Breast cancer	[118]
	Higher expression of MMP1 and MMP9 in brain-metastasizing tumor cells compared to primary and bone-metastasizing tumor cells		[119]
	MMP1 was part of specific gene expression signature for brain (and lung) metastatic relapse compared to relapse associated with bone, liver or lymph nodes		[120]
	Strong upregulation of MMP9 observed in reactive astrocytes localized in the immediate vicinity of extravasated tumor cells		[84]
	MMP1 degrades tight junctions of the BBB. Ectopic expression of MMP1 increases the neurotropic potential of tumor cells not associated with brain metastases		[121]
	EMT-promoting transcription factor Slug enhances MMP1 expression via directly binding to the promoter region on tumor cells		[122]
	Targeted knockdown of MMP1 in mice attenuated brain and lung metastasis formation in vitro and in vivo		[114, 121]
	Elevated serum level of MMP9 (but not MMP2) in patients with brain metastases. Both MMP2 and MMP9 significantly increased in patients’ cerebrospinal fluid	Multiple types	[115, 116]
	Elevated levels of the aldo–keto reductase AKR1B10 is associated with MMP2 and MMP9 expression via MEK/ERK signaling, facilitating BBB TJ degradation in vitro. Silencing of AKR1B10 downregulated MMP2 and MMP9 expression, suppressing both in vitro and in vivo tumor cell extravasation across the BBB	Lung cancer	[113]
	Invasion of the brain parenchyma by tumor cells is associated with MMP2 and MMP9. Tumor cell influence on microglia morphology induce the release of MMP2	Melanoma	[123–125]
Serine protease	Facilitates extravasation across an in vitro BBB model by disrupting junction complexes and causing apoptosis in ECs. The use of a serine protease inhibitor approximately halved the number of tumor cells able to migrate across an endothelial monolayer	Melanoma	[129]