Synthesis, maturation and extracellular release of procathepsin D as influenced by cell proliferation or transformation

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Abstract
The relationship between cell growth and intra‐ and extracellular accumulation of cathepsin D (CD), a lysosomal endopepti‐dase involved in cell protein breakdown, was examined in cultures of normal and transformed BALB/c mouse 3T3 fibro‐blasts 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. © 1995 Wiley‐Liss, Inc.
Funding Information
  • Ministero dell'Università e della Ricerca Scientifica (Rome), the Consiglio Nazionale delle Ricerche (Progetto A.C.R.O., Rome) and the Associazione Italiana pere la Ricerca sul Cancro