The ability of tumor cells to invade into the extracellular matrix has been attributed to the activity of cathepsins released by tumor cells or associated with the plasma membrane of tumor cells. Benign tumors are characterized by a continuous basal lamina separating the epithelium from the stroma. However, invasive carcinomas exhibit a disrupted extracellular lamina adjacent to the invading tumor cells in the stroma. Cathepsins secreted by invading tumor cells can degrade collagen and elastin, thereby destroying the basal laminar region. In normal cells, following their synthesis, cathepsins are transported into the lysosomal compartment. However, in tumor cells, instead of being transported into the lysosomal compartment, they are secreted into the surrounding medium. The presence of cathepsins in the extracellular compartment may be employed as an ideal independent prognostic factor to determine the clinical outcome of cancer chemotherapy.

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