Changes in Topoisomerase I Levels and Localization during Myeloid Maturation in Vitro and in Vivo1

Changes in topoisomerase I (topo I) levels and localization were exam ined during the course of granulocytic maturation in vitro and in vivo. Western blotting revealed that granulocytic maturation in DMSO-treated III.-6(1 human leukemia cells was accompanied by a 5-fold decrease in topo I polypeptide content. Consistent with this result, 3to 5-fold higher concentrations of the topo I poison camptothecin were required to stabi lize topo I-DNA adducts in DMSO-treated III,-60 cells compared to untreated cells. Northern blotting revealed that these changes occurred without any decrease in topo I message. Immunolocalization studies re vealed that these quantitative changes were accompanied by redistribu tion of topo I away from the nucleoli, where it was prominently accumu lated in untreated HL-60 cells, to a more uniform nuclear distribution in DMSO-treated cells. Similar changes occurred during granulocytic maturation in human marrow in vivo. Western blotting revealed that topo I levels in normal progranulocytes were 50% as high as those in HL-60 cells, levels in metamyelocytes were 35% as high as HL-60 cells, and levels in peripheral blood granulocytes were 5% as high as HL-60 cells. Two other poly peptides that are concentrated in nucleoli, poly(ADP-rÃ-bose) polymerase and B23/nucleophosmin, also decreased during the course of granulocytic maturation. These changes were accompanied by an alteration in topo I localization similar to that observed in HL-60 cells during the course of granulocytic maturation. Conversely, treatment of human lymphocytes with the mitogenic lectin concanavalin A resulted in a 3-fold increase in topo I polypeptide content concomitant with a prominent increase in the amount of nucleolar antigen. These observations not only provide a context for understanding the recent observation that topo I levels are higher in human leukemia specimens than in normal marrow but also raise the possibility that elevated topo I levels in other cells might reflect alterations in nucleolar structure and function.