Mechanism of the stimulatory effect of phorbol 12-myristate 13-acetate on cellular production of plasminogen activator.

The human Bowes melanoma cell line secretes a plasminogen activator identical to the extrinsic tissue plasminogen activator (EPA) and different from the urokinase-like plasminogen activators mostly found in human tumor lines. In the continuous presence of 100 ng/ml of phorbol 12-myristate 13-acetate (PMA) the 24-hr production of EPA was increased 5.3-fold (average). Preincubation of the cultures for a limited time period (optimally 3 to 6 hr) also resulted in an increase of the subsequent 24-hr production. EPA produced in the presence of PMA was serologically indistinguishable from that produced spontaneously and its molecular weight as defined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and fibrin-agar zymography was the same as that of spontaneously produced EPA. Treatment of the cells with actinomycin D inhibited PMA-induced EPA production. Also, RNA extracted from PMA-treated cells became enriched in mRNA for EPA. It is concluded that PMA acts by enhancing the transcription of the EPA gene. Cell-associated EPA levels were increased, even when tested as early as 3 hr after initiation of the PMA treatment, thus failing to support the view that increased EPA synthesis occurred as a result of depletion of the cellular pool.