ALTERED EXPRESSION AND ACTIVITY OF TOPOISOMERASES DURING ALL-TRANS-RETINOIC ACID-INDUCED DIFFERENTIATION OF HL-60 CELLS

Regulation of topoisomerase II (TOPO II) isozymes alpha and beta is in fluenced by the growth and transformation state of cells. Using HL-60 cells induced to differentiate by all-trans retinoic acid (RA), we hav e investigated the expression and regulation of TOPO II isozymes as we ll as the levels of topoisomerase I (TOPO I). During RA-induced differ entiation of human leukemia HL-60 cells, levels of TOPO I remained unc hanged, whereas the levels and phosphorylation of TOPO II alpha and TO PO II beta proteins were increased twofold to fourfold and fourfold to eightfold, respectively. The elevation of TOPO II (alpha and beta) pr otein levels and phosphorylation was apparent at 48 hours of treatment with RA and persisted through 96 hours. The increased level of TOPO I I beta protein was also detected in differentiated cells subsequently cultured for 96 hours in RA-free medium. Pulse chase experiments in ce lls labeled with S-35-methionine showed that the rate of degradation o f TOPO II beta 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 R A-treated cells. Whereas etoposide (1 to 10 mu mol/L) -induced DNA cle avage was not significantly different, apoptosis was significantly low er (P = .012) in RA-treated versus control cells after exposure to 10 mu mol/L etoposide, Consistent with unaltered levels of TOPO I, campto thecin (CPT) -induced DNA cleavage was similar in control ol RA-treate d cells. However, apoptosis after exposure to 1 to 10 mu mol/L CPT was significantly lower (P = .003 to P < .001) in RA-treated versus contr ol cells. Results suggest that TOPO II beta protein levels are posttra nscriptionally regulated and that degradation of TOPO II beta is decre ased during RA-induced differentiation. Furthermore, whereas the total level of TOPO II (alpha + beta) is increased with RA, the level of TO PO II catalytic activity and etoposide-stabilized DNA cleavage activit y remains unaltered. Thus, TOPO II beta may have a specific role in tr anscription of genes involved in differentiation with RA treatment. (C ) 1998 by The American Society of Hematology.