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

Altered Expression and Activity of Topoisomerases During All-Trans Retinoic Acid-Induced Differentiation of HL-60 Cells

Regulation of topoisomerase II (TOPO II) isozymes  and β is influenced by the growth and transformation state of cells. Using HL-60 cells induced to differentiate by all-trans retinoic acid (RA), we have investigated the expression and regulation of TOPO II isozymes as well as the levels of topoisomerase I (TOPO I). During RA-induced differentiation of human leukemia HL-60 cells, levels of TOPO I remained unchanged, whereas the levels and phosphorylation of TOPO II and TOPO IIβ proteins were increased twofold to fourfold and fourfold to eightfold, respectively. The elevation of TOPO II ( and β) protein levels and phosphorylation was apparent at 48 hours of treatment with RA and persisted through 96 hours. The increased level of TOPO IIβ protein was also detected in differentiated cells subsequently cultured for 96 hours in RA-free medium. Pulse chase experiments in cells labeled with 35S-methionine showed that the rate of degradation of TOPO IIβ protein in control cells was about twofold faster than that in the differentiated RA-treated cells. The level of decatenation activity of kDNA was comparable in nuclear extracts from control or RA-treated cells. Whereas etoposide (1 to 10 μmol/L) -induced DNA cleavage was not significantly different, apoptosis was significantly lower (P = .012) in RA-treated versus control cells after exposure to 10 μmol/L etoposide. Consistent with unaltered levels of TOPO I, camptothecin (CPT) -induced DNA cleavage was similar in control or RA-treated cells. However, apoptosis after exposure to 1 to 10 μmol/L CPT was significantly lower (P = .003 to P < .001) in RA-treated versus control cells. Results suggest that TOPO IIβ protein levels are posttranscriptionally regulated and that degradation of TOPO IIβ is decreased during RA-induced differentiation. Furthermore, whereas the total level of TOPO II ( + β) is increased with RA, the level of TOPO II catalytic activity and etoposide-stabilized DNA cleavage activity remains unaltered. Thus, TOPO IIβ may have a specific role in transcription of genes involved in differentiation with RA treatment.

© 1998 by The American Society of Hematology.

Altered Expression and Activity of Topoisomerases During All-Trans Retinoic Acid-Induced Differentiation of HL-60 Cells

Regulation of topoisomerase II (TOPO II) isozymes  and β is influenced by the growth and transformation state of cells. Using HL-60 cells induced to differentiate by all-trans retinoic acid (RA), we have investigated the expression and regulation of TOPO II isozymes as well as the levels of topoisomerase I (TOPO I). During RA-induced differentiation of human leukemia HL-60 cells, levels of TOPO I remained unchanged, whereas the levels and phosphorylation of TOPO II and TOPO IIβ proteins were increased twofold to fourfold and fourfold to eightfold, respectively. The elevation of TOPO II ( and β) protein levels and phosphorylation was apparent at 48 hours of treatment with RA and persisted through 96 hours. The increased level of TOPO IIβ protein was also detected in differentiated cells subsequently cultured for 96 hours in RA-free medium. Pulse chase experiments in cells labeled with 35S-methionine showed that the rate of degradation of TOPO IIβ protein in control cells was about twofold faster than that in the differentiated RA-treated cells. The level of decatenation activity of kDNA was comparable in nuclear extracts from control or RA-treated cells. Whereas etoposide (1 to 10 μmol/L) -induced DNA cleavage was not significantly different, apoptosis was significantly lower (P = .012) in RA-treated versus control cells after exposure to 10 μmol/L etoposide. Consistent with unaltered levels of TOPO I, camptothecin (CPT) -induced DNA cleavage was similar in control or RA-treated cells. However, apoptosis after exposure to 1 to 10 μmol/L CPT was significantly lower (P = .003 to P < .001) in RA-treated versus control cells. Results suggest that TOPO IIβ protein levels are posttranscriptionally regulated and that degradation of TOPO IIβ is decreased during RA-induced differentiation. Furthermore, whereas the total level of TOPO II ( + β) is increased with RA, the level of TOPO II catalytic activity and etoposide-stabilized DNA cleavage activity remains unaltered. Thus, TOPO IIβ may have a specific role in transcription of genes involved in differentiation with RA treatment.

© 1998 by The American Society of Hematology.

Inhibition of DNA topoisomerase II alpha gene expression by the p53 tumor suppressor

DNA topoisomerase II (topo II) is an essential nuclear enzyme involved in major cellular functions such as DNA replication, transcription, recombination, and mitosis. While an elevated level of topo II alpha is associated with cell proliferation, wild-type (wt) p53 inhibits the expression of various growth-stimulatory genes. To determine if p53 downregulates topo II alpha gene expression, a murine cell line, (10)1val, that expresses a temperature-sensitive p53 was utilized. The (10)1val cells had significantly lower levels of topo II alpha mRNA and protein following incubation for 24 h at 32 degrees C (p53 with wt conformation) than at 39 degrees C (p53 with mutant conformation). The effect of p53 on the human topo II alpha gene promoter was determined by using luciferase reporter plasmids containing varying lengths of the topo II alpha promoter transiently cotransfected into p53-deficient (10)1 cells together with wt or mutant p53 expression plasmids. Transcription from the full-length (bp -557 to +90) topo II alpha promoter was decreased 15-fold by wt p53 in a concentration-dependent manner, whereas mutant p53 exerted much weaker inhibition. Consecutive deletion of the five inverted CCAAT elements (ICEs) from the topo II alpha promoter reduced both the basal promoter activity and wt p53-induced suppression. Transcription of the minimal promoter (-32 to +90), which contains no ICE, was slightly stimulated by wt or mutant p53 expression. When point mutations were introduced into the most proximal ICE (-68), the inhibitory effect of wt p53 was alleviated and stimulation of topo II alpha expression resulted. Our study suggests that wt p53 functions as a transcriptional repressor of topo II alpha gene expression, possibly through a functional interaction with specific ICEs. Inactivation of wt p53 may reduce normal regulatory suppression of topo II alpha and contribute to abortive cell cycle checkpoints, accelerated cell proliferation, and alterations in genomic stability associated with neoplasia.

Phorbol ester-induced down-regulation of topoisomerase II alpha mRNA in a human erythroleukemia cell line. Evidence for a post-transcriptional mechanism

Tumor-promoting phorbol esters such as phorbol 12-myristate 13-acetate (PMA) are reported to induce megakaryocyte terminal differentiation of the erythroleukemia cell line K562. This differentiation is accompanied by the regulation of various gene products such as gamma-globin (Lumelsky and Forget, Mol Cell Biol 11: 3528-3536, 1991) and platelet-derived growth factor-beta (PDGF-beta) (Mäkelä et al., Mol Cell Biol 7: 3656-3662, 1987). PMA has also been found to regulate topoisomerase (topo) II alpha in other myeloid leukemia lines. The purpose of this study was to investigate whether PMA regulates topo II alpha in K562 cells and, if so, to identify the mechanisms responsible for this regulation. Northern blot analysis revealed that topo II alpha mRNA is down-regulated as is gamma-globin. This activity was not due to a generalized decrease in mRNA, as PDGF-beta message actually increased in response to PMA treatment. RNase protection assays confirmed the decline in the topo II alpha message. Transfection experiments with various topo II promoter CAT constructs extending to 2200 bp upstream of the ATG start site revealed regions that enhance and regions that inhibit CAT expression in the absence of PMA However, PMA did not affect this CAT expression. Run-on experiments using 5' and 3' human topo II cDNA probes confirmed that transcriptional initiation of the topo II gene was not affected by PMA, whereas that of c-myc did decrease. Therefore, the apparent decrease in topo II alpha mRNA in K562 cells upon their treatment with PMA appeared to be the result of a post-transcriptional mechanism.

Transfection of human topoisomerase II alpha into etoposide-resistant cells: transient increase in sensitivity followed by down-regulation of the endogenous gene

We have investigated the possibility of overcoming the resistance of human brain tumour cells (HBT20) to etoposide by transferring the normal human topoisomerase II alpha (H-topo II) gene into these cells. H-topo II in a mammalian expression vector containing a glucocorticoid-inducible mouse mammary tumour virus (MMTV) promoter was transfected into etoposide-resistant HBT20 cells (HBT20-hTOP2MAM). HBT20 cells transfected with pMAMneo vector alone served as control cells (HBT20-MAM). These were stable transfections. Following a 2 h dexamethasone treatment, H-topo II mRNA expression, protein production, etoposide-induced DNA-protein complex formation and sensitivity to etoposide were increased in HBT20-hTOP2MAM cells compared with control HBT20-MAM cells and with HBT20-hTOP2MAM cells not treated with dexamethasone. However, mRNA and protein levels and cell sensitivity returned to baseline when incubation with dexamethasone was continued for 24 h. This decrease from the 2 h values could not be explained by a loss of the MMTV promoter response to dexamethasone. (H-topo II alpha promoter)-(chloramphenicol acetyltransferase) constructs containing regions -559-0 and -2400-0 were significantly down-regulated in HBT20-hTOP2MAM cells treated for 24 h with dexamethasone compared with dexamethasone-treated control cells. H-topo II mRNA stability after 24 h of dexamethasone treatment was not altered compared with that in control cells. Our data indicate that the exogenously produced H-topo II may have a negative-feedback effect on the endogenous topoisomerase II promoter, causing down-regulation of the endogenous gene.