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 Mr 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.