TOPOISOMERASE II-ALPHA PROMOTER TRANSACTIVATION EARLY IN MONOCYTIC DIFFERENTIATION OF HL-60 HUMAN LEUKEMIA-CELLS

The cytotoxic efficacy of antitumor drugs targeted at DNA topoisomeras e II (topo II) in many cases varies in direct proportion to cellular t opo II content. To investigate the transcriptional control of the pred ominant alpha form of topo II, the 5' flanking region of the human top o II alpha gene (positions -562 to +90) was subcloned into a firefly l uciferase reporter plasmid and transiently transfected into HL-60 huma n leukemia cells, a line capable of monocytic differentiation after tr eatment with various agents. Early in phorbol-12-myristate-13-acetate (30 nM)-induced differentiation (18-24 hr after treatment), an unexpec ted 3-5-fold activation of topo II alpha gene promoter activity was ob served. Activation was observed in HL-60 cells and U-937 cells, but no t in HeLa human cervical carcinoma cells. Sodium butyrate (NaB) (0.4 m M) also led to activation (4-17-fold) of the topo II alpha promoter in HL-60 and U-937 cells, Promoter sequences between position -90 and po sition +90 mediated the inducing effects of NaB. This NaB-dependent pr omoter-reporter induction was partly mirrored by a transient similar t o 2-fold increase in endogenous topo II alpha enzyme. The stimulus for promoter activation could be partly attributed to a 2-fold increase i n DNA synthesis at 16 hr for NaB, but not phorbol-12-myristate-13-acet ate. Regardless of the primary stimulus for topo II alpha promoter tra ns-activation, it could be bypassed by treatment of HL-60 cells with N aB for 48 hr before transfection, revealing the expected 60-70% suppre ssion of topo II alpha promoter activity, Further study of topo II alp ha promoter down-regulation later in monocytic differentiation may ser ve as a model for elucidating the transcriptional mechanisms that may also be exploited by tumor cells expressing intrinsic or acquired resi stance to topo II-directed drugs.