Regulation of cathepsin D dependent on the phenotype of colon carcinoma cells

We have studied the intracellular trafficking of cathepsin D in different colon carcinoma cell populations: the HT‐29 cell line, composed of >95% undifferentiated cells; 2 subpopulations derived from this cell line, containing cells committed to differentiation into mucin‐secreting cells (HT‐29 MTX) or enterocyte‐like cells (HT‐29 G−) after confluence; and the Caco‐2 cell line, which spontaneously differentiates into enterocyte‐like cells after confluence. Post‐confluent undifferentiated HT‐29 cells and differentiated enterocyte‐like HT‐29 G− and Caco‐2 cells secrete significant levels of cathepsin D in culture medium, in contrast to post‐confluent differentiated mucin‐secreting HT‐29 MTX cells, which secrete this enzyme at a very low level. The intracellular content and the mRNA level of cathepsin D increase after confluence in the different cell types, particularly in Caco‐2 cells, which intensify the secretion of cathepsin D along with the differentiation process post‐confluence. Membrane‐associated mature cathepsin D was detected in HT‐29 cells but not in Caco‐2 cells. In the different types of cell, pro‐cathepsin D associates with the membrane concomitantly to its binding to an M, 72,000 protein. Membrane association persists after dissociation of the complex in HT‐29 cells but not in Caco‐2 cells. In the mucin‐secreting HT‐29 MTX cells, cathepsin D was immunolocalised to the membrane of mucin vacuoles localised under the brush border. Our results show that cathepsin D can be regulated differently in colon carcinoma cells, and this finding might have specific functional implications for each cell type. © 1996 Wiley‐Liss, Inc.