Metastasis March-April 1998 Figure 8 . Metastatic cancer cells must penetrate the connective tissue of the basal lamina and the stroma before they can exit an organ. The cells do this by employing proteases, protein enzymes that chew apart other proteins, such as the collagens in connective tissue. There, they help degrade cellular proteins to their constituent parts to be excreted or recycled to form new proteins.
Cancer cells seem to express cathepsin-B on their membrane surfaces as well.
Cathepsin-B is one of several proteases, some of which may be expressed and secreted by normal cell types, that help the cancer cell escape its parent tissue and enter the bloodstream.