MIS-SORTING OF PROCATHEPSIN-D IN METASTOGENIC TUMOR-CELLS IS NOT DUE TO IMPAIRED SYNTHESIS OF THE PHOSPHOMANNOSYL SIGNAL

Synthesis, processing and sorting of pro-cathepsin D (proCD), the prec ursor of a lysosomal protease involved in tumor-cell proliferation and invasion, were compared in various human tumor cell lines. In culture s of HepG2, HT29 and MCF7 cells the extracellular CD activity was up t o 10 times higher than in cultures of U937 and MG63 cells. Ammonium ch loride, which disrupts the pH-dependent receptor-mediated transport of lysosomal enzymes, exerted a differential effect on the secretion of CD activity in these cells. As judged by metabolic labeling and immuno precipitation, the secretion of CD synthesized in 24 hr in MG63 and U9 37 cells was enhanced 4- to 10-fold by ammonium chloride, while it was only slightly increased in HepG2, HT29 and MCF7 cells. In all tumor c ells examined, a portion of proCD was segregated into the endosomal ly sosomal pathway in the presence of ammonium chloride. However, the int ralysosomal maturation of CD was only inhibited by this drug in HT29 a nd MCF7 cells. The Golgi-associated processing of proCD, leading to th e synthesis of uncovered phosphomannosyl groups, proceeded with compar able efficiency in all tumor cells examined. These results suggest tha t hypersecretion of proCD in HepG2, HT29 and MCF7 cells is linked to t ransport mechanisms, as yet unidentified, which are independent of man nose-6-phosphate. (C) 1997 Wiley-Liss, Inc.