Procathepsin D expression correlates with invasive and metastatic phenotype of MDA-MB-231 derived cell lines. 
Procathepsin D (pCD) is a glycoprotein secreted abundantly by cancerous cells with a documented role in tumor development.  The levels of pCD in primary tumors are highly correlated with an increased incidence of metastasis.  Our earlier studies have shown that pCD exerts its effect on cancer cells through its activation peptide (AP) and involves both autocrine and paracrine modes of action.  In this study, we analyzed the expression and role of pCD in MDA-MB-231 and its derived cell lines 1833 and 4175 possessing discrete metastatic abilities.  Our results demonstrated a direct relationship between expression and secretion of pCD to the differential invasive potential of these cells.  Also, the cell lines responded to AP treatment by enhancing their invasive potential, proliferation and induction of secretion of various cytokines, suggesting that pCD plays a role in metastasis through its AP region.