Differentiation of U-937 promonocytic cells by etoposide and ICRF-193, two antitumour DNA topoisomerase II inhibitors with different mechanisms of action. 
We have compared the action on U-937 human promonocytic leukemia cells of two DNA topoisomerase II inhibitors, namely the epipodophyllotoxin etoposide and the bisdioxopiperazine ICRF-193. One hour pulse-treatment with 3 microM etoposide caused topoisomerase associated, primary DNA breakage, which was rapidly followed by apoptosis. By contrast, these effects were not observed upon pulse-treatment with 6 microM ICRF-193. However, continuous treatments with subcytotoxic concentrations of etoposide (0.15 microM) and ICRF-193 (0.3 microM) produced several similar effects, namely decreased cell proliferation, accumulation of cells at G2, increase in cell mass, and induction of differentiation. Under these conditions, etoposide produced a biphasic activation of protein kinase C, which consisted in an early transient activation (from hours 1 to 6) of the membrane-bound enzyme followed by a later activation (hour 48) of the total, membrane-bound and cytosolic enzyme. By contrast, ICRF-193 only provoked a late activation (from hours 72 to 96) of the total enzyme. When used at differentiation-inducing concentrations, both topoisomerase inhibitors caused a great stimulation of AP-1 binding activity, with maximum value at hour 12 in etoposide-treated cells and at hour 48 in ICRF-193-treated cells. By contrast, the binding activity of the NF-kappa(B) and EGR-1 transcription factors was little affected. It is concluded that topoisomerase II inhibitors may induce the differentiation of promonocytic cells, independently of their capacity to cause DNA strand breaks. However, there are other effects, such as the early activation of protein kinase C, which are probably derived from the production of primary DNA breakage by some anti-topoisomerase drugs.