COMMITMENT TO ERYTHROID-DIFFERENTIATION IN MOUSE ERYTHROLEUKEMIA-CELLS IS CONTROLLED BY ALTERATIONS IN TOPOISOMERASE II-ALPHA PHOSPHORYLATION

To explore the program of cell differentiation in Friend murine erythr oleukemia (MEL) cells, we used three clonal variants: phorbol 12-myris tate 13-acetate (PMA)-hypersensitive TS-19-101, PMA-resistant TR19-9, and hexamethylene bis-acetamide (HMBA)- and PMA-resistant DS19/R1, Aft er treating TS19-101 cells with HMBA, topoisomerase II (topo II) enzym atic activity was dramatically reduced, and cells became terminally di fferentiated, The initial reduction in activity was soon followed by r educed topo II alpha phosphorylation, but only later did the protein l evel drop significantly, PMA, which completely blocked HMBA-induced di fferentiation in TS19-101 cells, increased the phosphorylation of topo II alpha and restored the enzymatic activity to its original levels. Reduced topo II activity and phosphorylation were also evident in HMBA -treated TR19-9 cells, PMA Failed to restore topo II activity and phos phorylation to their original levels in TR19-9 cells, Predictably, the topo II activity and phosphorylation of DS19/R1 cells showed little c hange in response to HMBA or PMA treatment. Structural changes in chro matin became evident in sensitive cells 24 h after HMBA treatment, sug gesting that alterations in topo II alpha phosphorylation may control cell differentiation by altering nuclear architecture.