Expression and posttranslational fate of cathepsin D in HT-29 tumor cells depend on their enterocytic differentiation state

In the present work, we analyzed the variations in the expression and trafficking of cathepsin D (CD), a lysosomal endopeptidase, associated with the enterocytic differentiation of the human colon carcinoma HT-29 cell line. In spite of the fact that the abundance of CD mRNA was severalfold higher in undifferentiated HT-29 cells than in their enterocyte-like differentiated counterparts, the intracellular levels of CD activity and protein were found to be much higher in the latter. The kinetic of transport of newly synthesized proCD was different in the two cell populations: (a) full conversion of proCD into the lysosomal mature form required more than 24 h in differentiated cells, whereas it was almost complete within 8 h in undifferentiated HT-29 cells; and (b) the extracellular release of proCD was shown to occur more rapidly and to a higher degree in undifferentiated than in differentiated cells. Most of the secreted proCD contained phosphomannoses. Secretion of β-hexosaminidase activity doubled, whereas that of CD activity was unchanged, upon vacuolar alkalinization with ammonium chloride or chloroquine. Inhibition of the lysosomal-autophagic degradative pathway resulted in the accumulation of proCD molecules in undifferentiated HT-29 cells. Altogether these data suggest that: (a) the expression and the posttranslational fate of CD in HT-29 colon cancer cells are largely affected by the state of their enterocytic differentiation; and (b) in this cell line the acid-dependent mannose 6-phosphate receptor pathway is, at best, little involved in the trafficking of CD.