Table 1.

Cysteine cathepsins in breast cancer

Cathepsin	Main cellular source	Cellular localization	Specific role in metastasis to	Breast cancer subtype	Expression in increasing grade and stage	References
Cathepsin B	Tumor cells Macrophages Fibroblasts Endothelial cells Myoepithelial cells	Lysosomal Extracellular	Lung Bone	IBC HER2 +  ER +  PR +	Increases	[56, 58, 60–83]
Cathepsin C	Tumor cells (that metastasized to the lung) Leukocytes Fibroblasts	Lysosomal	Lung	ER +  PR +  HER2 +  TNBC	Unknown	[84–87]
Cathepsin K	Tumor cells (that metastasized to the bone) Myofibroblasts	Lysosomal Extracellular	Bone	ER +  PR +  HER2 +  TNBC	Increases	[38, 91–99]
Cathepsin L	Tumor cells Macrophages Myofibroblasts	Lysosomal Extracellular	Lung	ER +  PR +  HER2 +  TNBC	Increases	[56–58, 61, 66, 67, 95, 100–106, 108]
Cathepsin O	Tumor cells	Lysosomal	Unknown	ER +	Unknown	[109–112]
Cathepsin S	Tumor cells Macrophages	Lysosomal Extracellular	Brain	TNBC ER +  PR +	Unknown	[30, 33, 69, 95, 105, 113–117]
Cathepsin V	Tumor cells Stromal fibroblasts	Lysosomal	Unknown	ER +  HER2 +	Increases	[33, 118–121]

Shown are the cellular sources, cellular localization, specific roles, associated breast cancer subtypes, and the influence of grade and stage on expression for the different cysteine cathepsins discussed in this review

Abbreviations: IBC, inflammatory breast cancer; HER2, human epidermal growth factor receptor 2, ER, estrogen receptor; PR, progesterone receptor; TNBC, triple negative breast cancer