Molecular analysis of a potentially phorbol-regulatable region of the human topoisomerase II alpha gene promoter.

Topoisomerase II alpha (topo II alpha) mRNA was down-regulated to a greater extent in 2 human leukemia HL-60 cell lines sensitive to PMA-induced terminal differentiation than in their non-differentiating daughter lines following exposure to PMA (Cancer Res., 50: 7116-7122, 1990; Biochem. Pharmacol., in press). The sequence of the topo II alpha promoter (ATG upstream to -650) in all four cell lines was identical to that of a human lymphocyte genomic clone and to that of the previously published sequence from a human placenta clone (J. Biol. Chem., 267: 18961-18965, 1992). Putative transcriptional start sites were identical in one sensitive/resistant pair. In the other pair, a methylated site was identified between positions -242 and -580 within the -650 bp promoter region of the resistant daughter cell only. The identity of the sequence from all four cell lines indicates that mutations in the topo II alpha gene promoter of PMA-resistant cells cannot explain the absence of topo II alpha mRNA down-regulation following PMA treatment. Altered methylation patterns may, however, contribute to the reduced decrease in topo II alpha gene expression in one PMA-resistant line.