CTSB

CTSB Full Name

cathepsin B

CTSB Introduction

Cathepsin B (CTSB) is a ubiquitous, lysosomal cysteine protease belonging to the papain superfamily. Initially synthesized as an inactive preproenzyme in the rough endoplasmic reticulum, it undergoes sequential proteolytic processing to become a fully mature, active enzyme within the acidic environment of the lysosome. Physiologically, CTSB plays a fundamental role in intracellular protein degradation, maintaining general cellular homeostasis, and facilitating constant protein turnover. Notably, it is also actively involved in specific physiological processes such as antigen processing in the immune system, thyroxine production in the thyroid gland, and bone resorption by osteoclasts. From a structural and biochemical perspective, CTSB is highly unique among human cathepsins. It possesses a flexible structural element known as the "occluding loop," which partially blocks the active site cleft. This structural feature allows CTSB to function not only as an endopeptidase (cleaving internal peptide bonds) but also as a peptidyldipeptidase (an exopeptidase that removes dipeptides from the C-terminus of substrates). This dual catalytic capability makes CTSB exceptionally efficient at dismantling a wide variety of protein substrates.

Figure 1. Movement of the occluding loop of cathepsin B in different states of the enzyme. (Source: Redzynia I, et al. 2008)

While indispensable for normal cellular maintenance, the dysregulation, overexpression, and aberrant secretion of CTSB are heavily implicated in numerous severe pathologies. In the context of oncology, CTSB is frequently overexpressed in various malignancies, including breast, colorectal, and brain cancers. When secreted into the extracellular space, CTSB actively degrades key components of the extracellular matrix (ECM) and basement membrane, such as laminin, fibronectin, and collagen. This proteolytic remodeling of the tumor microenvironment directly facilitates tumor cell invasion, angiogenesis, and subsequent metastasis.

Beyond cancer, abnormal CTSB activity is intimately linked to neurodegenerative disorders, most notably Alzheimer's disease, where it is thought to participate in the neurotoxic processing of the amyloid precursor protein. Furthermore, the leakage of CTSB from lysosomes into the cytosol is a known trigger for lysosomal-mediated cell death pathways and inflammasome activation, contributing to chronic inflammatory conditions like rheumatoid arthritis and osteoarthritis. Because of its prominent role in these destructive diseases, CTSB continues to be rigorously investigated as a high-value diagnostic biomarker and a promising therapeutic target for highly specific protease inhibitors.

Alternate Names for CTSB

CTSB; cathepsin B; APPS; CPSB; cathepsin B1; APP secretase; cysteine protease; amyloid precursor protein secretase;

• 1. Aggarwal N, et al. Cathepsin B: multiple roles in cancer. Proteomics Clin Appl. 2014 Jun;8(5-6):427-37.
  2. Redzynia I, et al. Displacement of the occluding loop by the parasite protein, chagasin, results in efficient inhibition of human cathepsin B. J Biol Chem. 2008 Aug 15;283(33):22815-25.