Role of Cysteine Cathepsins in Extracellular Proteolysis

Abstract

Cysteine cathepsins are lysosomal proteases with housekeeping as well as highly specialized functions. Although their activities are optimal at lysosomal acidic and reducing conditions, cathepsins can significantly contribute to the degradation of the extracellular matrix. This may happen under physiological conditions as in cathepsin K-mediated bone resorption or under pathological conditions. Extracellular matrix degradation can occur extracellularly by the secretion of cathepsins or intracellularly following the endocytosis of matrix material. Under physiological conditions, the extracellular matrix is safeguarded against cathepsin activities by its neutral pH, oxidative environment, and high levels of potent endogenous cathepsin inhibitors. However, these barriers can be overcome by pericellular acidification and pathophysiologically reduced anticathepsin concentrations. Whereas matrix metalloproteases are primarily responsible for the homeostasis of the extracellular matrix, cysteine proteases contribute to its destruction under disease conditions. The development of cathepsin inhibitors as anti matrix-degrading drugs appears to be a successful strategy.