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.
Loflin
P T
PT
Department of Clinical Investigation, University of Texas M.D. Anderson Cancer Center, Houston 77030, USA.
Altschuler
E
E
Hochhauser
D
D
Hickson
I D
ID
Zwelling
L A
LA
D000818:Animals; D002415:Cats; D004264:DNA Topoisomerases, Type I / Q000187:drug effects*; D018076:DNA, Complementary; D015536:Down-Regulation / Q000187:drug effects*; D006801:Humans; D004915:Leukemia, Erythroblastic, Acute / Q000188:drug therapy*; D010703:Phorbol Esters / Q000494:pharmacology*; D012333:RNA, Messenger / Q000378:metabolism; D014407:Tumor Cells, Cultured
D018076:DNA, Complementary; D010703:Phorbol Esters; D012333:RNA, Messenger; D004264:DNA Topoisomerases, Type I