Transfected cathepsin D stimulates high density cancer cell growth by inactivating secreted growth inhibitors.
Cathepsin D, a lysosomal protease, is overexpressed in primary breast cancer and associated with increased risk of metastasis. We have shown previously by transfection in rat tumor cells that overexpression of cathepsin D increased both experimental metastasis in nude mice and in vitro proliferation under low-serum conditions. In this study, we used the transfected cell lines to investigate the mechanism by which cathepsin D prevents density-dependent arrest of cell proliferation. This effect was not associated with a general alteration of cell-substratum or cell-cell adhesiveness. As shown by coculture and conditioned media experiments, control cells reaching saturation density released inhibitory activity that was able to prevent the growth of control or cathepsin D transfectants and decreased the cloning efficiency of normal rat kidney fibroblasts in agar. By contrast, in media from two cathepsin D-transfected cell lines, this inhibitory activity was markedly reduced. Cathepsin D overexpression did not affect cell sensitivity to the inhibitor but modified the secretion of several proteins. The increase in cell density appeared to be due to intracellular maturation of cathepsin D since it was reversed by amine treatment that neutralizes the pH of acidic compartments within the cells. Moreover, the addition of secreted pro-cathepsin D was unable to increase the saturation density of control clones. Finally, the inhibitory factor was partially characterized as a heat-labile, secreted protein. We conclude that cathepsin D overexpression increases the growth of cancer cells to a higher density via an intracellular mechanism, leading to a decreased secretion of growth inhibitor(s).
Institut National de la Santé et de la Recherche Médicale, Unité Hormones et Cancer, U 148, Montpellier, France.
E
E
Liaudet
D
D
Derocq
H
H
Rochefort
M
M
Garcia
D000818:Animals; D002402:Cathepsin D / Q000096:biosynthesis*; D002448:Cell Adhesion / Q000235:genetics; D002452:Cell Count; D002455:Cell Division / Q000235:genetics; D006131:Growth Inhibitors / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051379:Mice; D008819:Mice, Nude; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D013268:Stimulation, Chemical; D014162:Transfection / Q000235:genetics*; D014407:Tumor Cells, Cultured
D006131:Growth Inhibitors; D009363:Neoplasm Proteins; D002402:Cathepsin D