Phorbol ester activation of a proteolytic cascade capable of activating latent transforming growth factor-β -: A process initiated by the exocytosis of cathepsin B

12-O-Tetradecanoylphorbol-13-acetate (TPA) suppresses the proliferation of the human breast epithelial cell line MCF10A-Neo by initiating proteolytic processes that activate latent transforming growth factor (TGF)-)-beta in the serum used to supplement culture medium. Within 1 h of treatment, cultures accumulated an extracellular activity capable of cleaving a substrate for urokinase-type plasminogen activator (uPA) and tissue plasminogen activator (tPA). This activity was inhibited by plasminogen activator inhibitor-1 or antibodies to uPA but not tPA. Pro-uPA activation was preceded by dramatic changes in lysosome trafficking and the extracellular appearance of cathepsin B and beta-hexosaminidase but not cathepsins D or L. Co-treatment of cultures with the cathepsin B inhibitors CA-074 or Z-FA-FMK suppressed the cytostatic effects of TPA and activation of pro-uPA. In the absence of TPA, exogenously added cathepsin B activated pro-uPA and suppressed MCF10A-Neo proliferation. The cytostatic effects of both TPA and cathepsin B were suppressed in cells cultured in medium depleted of plasminogen/ plasmin or supplemented with neutralizing TGF-beta antibody. Pretreatment with cycloheximide did not suppress the exocytosis of cathepsin B or the activation of pro-uPA. Hence, TPA activates signaling processes that trigger the exocytosis of a subpopulation of lysosomes/endosomes containing cathepsin B. Subsequently, extracellular cathepsin B initiates a proteolytic cascade involving uPA, plasminogen, and plasmin that activates serum-derived latent TGF-beta.