Stimulation of DNA synthesis by transient exposure of cell cultures to TPA or polypeptide mitogens: induction of competence or incomplete removal?

A transient exposure of cell cultures to 12-0-tetradecanoyl-phorbol-13-acetate (TPA) is sufficient to stimulate DNA synthesis during a subsequent incubation in TPA-free medium. We show that (1) a substantial fraction of TPA remains bound to cultures following a transient exposure to TPA and thorough washing, (2) the ability of TPA to induce DNA synthesis is a function of the amount of TPA bound to cell cultures irrespective of whether it is incubated continuously with cultures or transiently exposed to cultures under various conditions, and that (3) a transient exposure of cultures to phorbol-12-13-dibuytrate (PDB), a mitogenic phorbol ester which binds reversibly to cell cultures, does not stimulate DNA synthesis during a subsequent incubation in PDB-free medium. Therefore the persisting effects of TPA are due to it binding to cultures in a manner resistant to washing and not due to the induction of a stable cellular change prerequisite for mitogenesis. Further, we show that certain combinations of polypeptide growth factors induce DNA synthesis in the absence of any such stable cellular change. Evidence is also presented that the persisting effects on DNA synthesis following transient exposure of cultures to other polypeptide growth factors (e.g., platelet-derived growth factor) reflect tenacious binding rather than induction of a lasting biological event.