The cell surface binding, endocytosis, and lysosomal routing of
procathepsin D (procath-D) in
cancer cells are mostly independent of the
mannose-6-phosphate (M6P) receptors. In an attempt to define the receptor involved, we intracellularly cross-linked procath-D with a 68-kDa
protein that we identified with specific
antibodies as prosaposin in human breast and
ovarian cancer cell lines. In
cancer cells, this
protein-
protein interaction was resistant to
ammonium chloride or M6P treatment, indicating that it was independent of the M6P receptors. A similar interaction also occurred in the
breast cancer cell culture medium between the secreted prosaposin and procath-D. Since these two precursors can be endocytosed, we then determined whether they were interacting with the same
cell surface receptor. In fibroblasts, we confirmed that the endocytosis of these two
proteins was different since it was generally mediated by the M6P receptors for procath-D and mostly by LRP (
LDL receptor-related protein) for prosaposin. In
breast cancer cells, prosaposin endocytosis was not detected, in contrast to procath-D endocytosis, suggesting that the majority of procath-D is not internalized as a complex with prosaposin. Moreover, RAP (receptor-associated
protein), a ligand inhibiting LRP-mediated endocytosis, prevented internalization of prosaposin in 49-F rat fibroblasts, but did not affect procath-D M6P-independent internalization in MDA-MB231 cells. We conclude that in
breast cancer cells, even though procath-D interacts intracellularly and extracellarly with prosaposin, it is endocytosed independent of prosaposin by a receptor different from the M6P receptors and the LRP.