Cathepsins are protease enzymes, categorized into multiple families. They can be serine protease, cysteine protease, or aspartyl protease. There were about 15 classes of cathepsins in humans. Cathepsin E is expressed on a broad range of immune cells, cathepsin K on skin fibroblasts, and cathepsin L only in the placenta.  Cathepsin A causes the inactivation of bioactive peptides such as bradykinin, substance P, oxytocin, angiotensin I and endothelin-I. Cathepsin A can inhibit autophagy. Cathepsin B promotes amyloid plaque, and various carcinomas. This enzyme is instrumental in both basal and EGF (epidermal growth factor)-stimulated lung cancer cell migration. Prorenin, the precursor of kidney-secreted hormone renin, can be activated by cathepsin B. 
Cathepsin B from amoeba can cleave several human proteins including immunoglobulins (IgA, IgG, IgM), hemoglobin, collagen, fibronectin, and albumin. Cathepsin D cleaves fibronectin and laminin. Cathepsin D can express on desmosomes, the intercellular junctions, causing desquamation. Cathepsin E is frequently implicated in antigen processing via the MHC class II pathway. Cathepsin K is highly effective in degrading collagens. Cathepsin K also degrades gelatin, the latter being a hydrolysis product of collagen. Cathepsin L degrades fibronectin, insulin receptor, and insulin-like growth factor 1 receptor (IGF-1R). Cathepsin V (or L2), an elastase, is involved in cancer invasion and metastasis. Cathepsins degrade lowdensity lipoprotein (LDL-P) and attenuate cholesterol efflux from macrophages, leading to foam cell formation. The foam cells are responsible for atherosclerosis and coronary artery disease. Fibrosis and cancer, among other inflammatory diseases, are the resultant of perturbed ECM.


1.Patel S,et al. Biomed Pharmacother. 2018 Sep;105:526-532.