| 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. Proteases such as cathepsin-B are normally housed in
membrane-bound vesicles, called lysosomes, inside the cell.
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.
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