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Cathepsin D (CD), a lysosomal aspartic protease present in mammary tissue and milk in various molecular forms, is also found in the incubation medium of mammary acini in molecular forms that are proteolytically active on prolactin at a physiological pH. Because prolactin controls the vesicular traffic in mammary cells, we studied, in vivo and in vitro, its effects on the polarized transport and secretion of various forms of CD in the rat mammary gland. CD accumulated in vesicles not involved in endocytosis in the basal region of cells. Prolactin increased this accumulation and the release of endosomal active single-chain CD at the basal side of acini. The CD-mediated proteolysis of prolactin, leading to the antiangiogenic 16-kDa form, at a physiological pH, was observed only in conditioned medium but not milk. These data support the novel concept that an active molecular form of CD, secreted at the basal side of the mammary epithelium, participates in processing blood-borne prolactin outside the cell, this polarized secretion being controlled by prolactin itself.

The relationship between cell growth and intra- and extracellular accumulation of cathepsin D (CD), a lysosomal endopeptidase involved in cell protein breakdown, was examined in cultures of normal and transformed BALB/c mouse 3T3 fibroblasts grown at various cell densities. In crowded cultures of normal 3T3 cells (doubling time, Td, 53 hr) intracellular CD activity was 2-fold higher than in sparse, rapidly-growing (Td, 27 hr) cultures. In uncrowded (Td, 18 hr) and crowded (Td, 32 hr) cultures of benzo[a]pyrene-transformed cells intracellular CD levels were one third and two thirds, respectively, of those measured in hyperconfluent 3T3 cultures. Regardless of cell density, SV-40-virus-transformed cells (Td, 12 hr) contained one third of CD levels found in hyperconfluent 3T3 cells. Both transformed cell lines released into the medium a higher proportion of CD, compared with their untransformed counterpart, yet the amount secreted was not sufficient to account for the reduced intracellular level of the enzyme. Serum withdrawal induced a marked increase of both intra- and extracellular levels of CD activity. In both normal and virally or chemically transformed 3T3 cells CD comprised a precursor (52 kDa) and processed mature polypeptides; the latter were mostly represented by a 48-kDa peptide, but a minor part was in a double-chain form (31 and 16 kDa respectively). The proportion of mature enzyme vs. precursor was much higher in confluent, slowly-growing cells than in fast-growing cells, whether normal or transformed. In the latter, conversion of mature 48-kDa peptide into the double-chain form occurred more efficiently.

Both freshly-isolated rat hepatocytes and Morris hepatoma 7777 cells synthesized cathepsin D as a precursor that was either processed intracellular to smaller mature forms or secreted into the medium. The pattern of mature enzyme forms was different in the 2 cell types. In addition, the relative amount of precursor secreted was much higher for hepatoma cells. Monensin strongly enhanced the secretion and also impaired the intracellular transport-linked maturation of procathepsin D in hepatocytes, while it markedly inhibited intracellular maturation and only slightly increased secretion of the pro-enzyme in hepatoma cells. Ammonium chloride influenced the intralysosomal segregation and maturation of procathepsin D in hepatocytes but not in hepatoma cells. Our observations indicate that (i) the lysosomal segregation of cathepsin D was less efficient and its fractional secretion higher in hepatoma cells than in hepatocytes; (ii) in the 2 cell types, delivery to lysosomes and processing of procathepsin D were differently sensitive to increases in the vacuolar pH.